Neuroendocrine physiology of the early and late menopause.

Hormonal integration of the reproductive system is dramatically affected by reproductive aging. The progressive loss of ovarian follicles with normal aging is accompanied by an initial decrease in inhibin B and a concomitant increase in follicle-stimulating hormone. Subsequently, inhibin A and progesterone decrease, where as estradiol levels are maintained and often increase. In the late reproductive stage, cycles remain regular whereas the early and late menopausal transition are characterized by irregular cycles and often dramatic swings in estradiol and gonadotropin levels. Studies in younger and older postmenopausal women suggest that there are age-related changes in the neuroendocrine axis that are independent of the changing ovarian hormonal milieu of the menopausal transition but may contribute to the end of reproductive life.